Precipitation Definition: From Clouds to the Ground

Three meanings of precipitation

1. Weather. Water that condenses in clouds and falls to earth in any form. This is the meaning most readers want and the one taught in school geography.
2. Chemistry. The process in which a dissolved substance comes out of a solution as a solid, called a precipitate. A cloudy solid suddenly appearing when two clear liquids are mixed is a classic sign.
3. Haste. An old, formal sense meaning sudden or reckless hurry: He acted with precipitation and regretted it. You meet it mostly in older books and legal or diplomatic language.

All three share the Latin idea of something falling or rushing headlong, whether water from the sky, a solid out of a liquid, or a person into a decision.

The precipitation definition in weather and climate

Water vapour rises, cools and condenses into tiny droplets or ice crystals that form clouds. When those droplets or crystals grow heavy enough, they fall. That falling water is precipitation, one stage in the water cycle along with evaporation, condensation and collection in rivers, lakes, soil and oceans.

Fog and dew are not usually counted, because they form at or near the ground rather than falling from clouds.

How precipitation is measured

Weather stations catch precipitation in a rain gauge, an open container that records how deep the water would be if none ran off or soaked in. Totals are given in millimetres in India and most countries, and in inches in the United States. Snow is often melted and reported as its liquid equivalent so that it can be compared with rain.

Climate scientists add these readings over months and years to map wet and dry regions. In much of India, the monsoon months bring most of the year's rainfall, so precipitation figures for June to September are closely watched by farmers and planners alike. Patterns of precipitation shape agriculture, water supply and the health of every ecosystem.

Precipitation in chemistry

When two solutions are mixed and their ions combine into a compound that does not dissolve, that compound appears as a solid. For example, mixing clear silver nitrate solution with clear sodium chloride solution produces a white solid, silver chloride, which settles to the bottom. That solid is the precipitate, and the reaction is a precipitation reaction.

Chemists use this to detect certain substances, to purify compounds and to remove unwanted materials from water. The verb is precipitate: the salt precipitated out of the solution as it cooled. Related ideas such as density explain why the solid sinks.

Precipitation in sentences

• The forecast shows a 70 percent chance of precipitation after lunch, so take an umbrella.
• Annual precipitation in the desert is less than the amount some coastal towns get in a single week.
• In the lab, precipitation began the moment we added the second reagent, leaving yellow specks in the tube.
• Higher ground often gets more precipitation because rising air cools and forms clouds.
• The committee warned against precipitation in a matter that needed careful thought.

Word history and related words

The noun entered English in the late 1400s, at first for a casting down. It comes from Latin praecipitatio, the act of falling headlong, from praeceps, headlong or steep. The haste sense dates from about 1500, the alchemy sense of a solid separating from a liquid also goes back to that early period, and the weather sense dates from the 1670s.

Watch these relatives:

• Precipitate as a verb means to cause something to happen suddenly (the crisis precipitated a change) or to form a solid from a solution. As an adjective it means hasty.
• Precipitous means very steep, or a sudden, sharp fall: a precipitous drop in sales.

Find more terms like these in our science and ideas collection.